Positive-Negative Feedback Loop between miR-197 and IL-17A Signaling in Human Keratinocytes

Psoriasis is a chronic inflammatory skin disorder which results from pathological interactions between activated immunocytes and keratinocytes. Recent studies implicated the role of IL-17 and IL-22, secreted from Th17 and Th22 in the generation and propagation of the psoriatic plaque. Previously, we and others have shown that the expression of miR-197 is significantly decreased in psoriatic lesions. We further showed that miR-197 targets IL-22RA1 and that ectopic expression of miR-197 prevent IL-22 induced proliferation and migration of keratinocytes. Since the 3'UTR of the IL17RA subunit mRNA contains a putative binding site for miR-197, our aim was to expand our understanding of the miRNA-mediated crosstalk between immunocytes and keratinocytes by studying the effect of miR-197 expression on IL-17A signaling pathway. Luciferase reporter assays along with Western blot analysis revealed that miR-197 directly targets the 3'UTR of IL17RA. Furthermore, ectopic expression of miR-197 led to a significant decrease in IL-17A-induced expression of CCL20, a known downstream effector of IL-17A. Interestingly, the addition of IL-17A to keratinocytes led to a rapid and transient increase in the expression of miR-197. Chromatin immuno-precipitation assays showed that keratinocytes' treatment with IL-17 leads to C/EBP binding to the promoter region of miR-197, and that the expression level of miR-197 is directly proportional to the extent of C/EBP binding to the promoter. Moreover, following treatment with IL-17A, the histone acetylation pattern at the miR-197 promoter turns to become characteristic of transcribed chromatin. Taken together, our results suggest that a positive-negative feedback loop exists between IL-17A and miR-197 in keratinocytes; the cytokine induces the binding of C/EBPα to miR-197 promoter sequences, enhances miR-197 expression that negatively attenuates IL-17 receptor and decreases the input along the IL-17A pathway. Our work suggests that in psoriasis, decreased expression of miR-197 may prevent the miR-197-induced attenuation of the IL-17 cascade, leading to its over-activity.